Electron multiplier



June 21, 1960 A. A. ROTOW ETAL ELECTRON MULTIPLIER Filed 001 2, 1957 till) INVENTORS Auxmnzn A. Ru'rnw 5 BY EUGENE A.DYMAEEK MM 4 Ems ELECTRON MULTIPLIER Alexander A. Rotow and Eugene A. Dymacek, Lancaster,

Pa., assignors to Radio Corporation of America, a corporation of Delaware Filed Oct. 2, 1957, Ser. No. 687,822

5 Claims. (Cl. 313-103) This invention relates to electron multipliers and paratent ticularly to an improved electron multiplierfor use in t a photosensitive type of tube.

r One of the problems in the selection of a material for use as an electron multiplier structure, which is to be used in a photosensitive tube, is the problem of selecting a material that provides a uniform secondary electron emission from all emitting areas of j the multiplier. When the secondary electron emission is not uniform from all emitting areas, the signals resulting from the emission tend to provide a shading or non-uniformity of the original signal.

1 Another factor in the selection of materials for use as an electron multiplier is that secondary electron emission must be constant during the normal tube life. When the secondary electron emission is not constant, the signal from the multiplier will vary with the age of the device, which normally results in a relatively inefficient electron multiplier after the device has been used for a particular length of time.

The problems of obtaining a uniform, and of maintaining a constant, electron emission from a secondary electron multiplier are particularly pronounced in certain photosensitive tubes, such as pickup tubes and photo tubes. One particular type of photosensitive tube in which these problems are of particular importance is the type of photosensitive pickup tube which is used in television. If a non-uniformity of the secondary electron emission exists in the electron multiplier within the pickup tube, the output signal from the tube is distorted by the superposition of spurious signals introduced by the nonuniformity in secondary emission. The spurious signals are known as bad shading by those skilled in the art.

The causes of bad shading in a pickup tube are diificult to determine. However, in accordance with this invention, it has been found that one of the more important factors producing bad shading is that of dynode burn."

Dynode burn is a change in the secondary electron emission of limited areas of the first dynode surface durthe operation of the tube. The cause of this change in secondary electron emission has not been clearly understood prior to this invention. However, in accordance with one of the aspects of this invention, it has been found that the dynode burn is caused partially by the simultaneous action of electron bombardment of the first dynode, in conjunction with high ambient temperatures such as that caused by heat radiated from the cathode of an adjacent electron gun.

Dynode burn problems are amplified in television pickup tubes due to the fact that, during the formation of the photocathode in a pickup tube, first oxygen and then cesium is introduced into the tube. Therefore, there is a possibility that areas of the surface of the dynode material become oxidized, then this oxide binds to and/ or absorbs by adhesion some of the cesium that is present in the tube. Under the joint influence of heat and electron bombardment, during the subsequent operation of the tube, some of the cesium might be removed from ZMZJBZ the surface thus musing a decrease in the secondary electron emission from an area of the dynode.

It is therefore an object of this invention to provide a new andimproved dynode for an electron multiplier characterized, by its uniform and constant secondary electron emission during tube operation.

It is another object of this invention to provide an improved first dynode for a secondary electron multiplier characterized by its stability under conditions of tube processing, manufacturing and operation.

, These and other objects are accomplished in accordance with this invention by making the first dynode of a material that is stable with respect to the photosensitive materials usedin the tube, and of a material that has a sufiiciently high, constant, secondary electron emission. The material which has been found to meet these requirements'is chromium.

This invention will be more clearly understood by reference to the accompanying single sheet of drawings wherein:

Fig. l is a longitudinal cross section of a pickup tube of the type incorporating this invention; and,

Fig. 2 is an enlarged sectional view of the first dynode structure of the tube of Fig. 1.

Although the invention is applicable to all types of tubes wherein an electron multiplier arrangement is used, it is particularly applicable to photosensitive tubes using an electron multiplier structure. Furthermore, for simplicity of illustration, the invention will be described with particular reference to a pickup tube of the image orthicon type as an example.

Referring to Fig. 1, there is shown a sectional view of an image orthicon type pickup tube 10. The tube comprises an electron gun 16 in one end of an evacuated envelope 11. The electron gun 16 is of a well-known type and further description thereof is not deemed necessary. The electron gun 16 is for the purpose of producing an electron beam 14 which is accelerated, by accelerating electrodes, toward a target electrode 18 with potentials such as those shown in 'Fig. 1. The target electrode 18 is an insulator, e.g. glass, and is supported transverse to the electron beam 14. Mounted immediately adjacent to the target 18 is a decelerating electrode 20 which brings the velocity of the electron beam 14 to substantially zero in front of the targetsurface. The electron beam 14 will land on the target and drive the front or scanned surface of the target 18 to cathode potential after which the electron beam is reflected by the target as a return electron beam-15. The fields, produced by a deflection yoke 1-9, and focus coil 21, which cause the electron beam to scan the target also cause the return beam 15 to scan the first dynode electrode.

At the opposite end of the tube 10, there is formed a photocathode 22 which emits electrons in proportion to the amount of light focused thereon from a scene to be reproduced. The photocathode 22 normally includes an oxidized and cesiated silver alloy or a film of antimony activated with small amounts of a plurality of alkali metals.

The electrons from the photocatbode 22 are accelerated and focused onto the back of the target 18, during tube operation, to produce, by secondary emission, a charge pattern which appears on the side of the target 18 toward the electron gun 16. When the electron beam 14 is scanned over the front surface of the target 18, the beam is reflected from the areas of the target 13 that are not charged by the photocathode 22 since these areas are at cathode potential. The beam 14 lands on the areas of the target 18 that are charged by the photocathode until the charge pattern is neutralized by the electron beam 14. Once the charge is removed, the balance of the beam is reflected toward the electron gun 16. Thus, the return Patented June 21, 1960 electron beam 15 is in proportion to the amount of charge on the target 18.

The electron beam returns toward the electron gun 16 and scans over a first dynode 24 that surrounds and forms an aperture 17 in the first accelerating electrode for the primary electron beam. The return electron beam 15 is multiplied by secondary electron emission of the first dynode 24 and is directed into anelectron multiplier 26. Generally, the electrons from the first dynode will land with a scattered distribution over the area of the second dynode so that areas of the second dynode that are of a non-uniform secondary emission are not a pronounced problem. The electron multiplier 26 may be of any type such as that disclosed in U.S. Patent 2,433,941 to P. K. Weimer. The modulated return electron beam is converted into output video signal voltages from a collector electrode of the electron multiplier 26.

During the manufacturing process of tube 10, certain reactive materials, such as antimony, cesium and oxygen, are introduced within the tube to form the photocathode 22. Some of these materials may land on the first dynode 24 either during manufacturing or subsequently during tube operation since the materials may be occluded in the electrodes or the envelope walls and be gradually released.

The proximity of the first dynode electrode 24 to the electron gun 16 should be noted. Due to this proximity, the first dynode electrode 24 will be heated by heat radiated from the heater of the electron gun 16. Due to the presence of the reactive materials and the heat from the cathode, certain reactions have occurred in the prior art tubes which have resulted in dynode burn or change in the secondary emission of the first dynode surface. As was explained, dynode burn produces spurious signals in the output signal as the return beam 15 scans over the non-uniform secondary emitting areas of the first dynode.

In accordance with this invention, the first dynode 24, which is shown more clearly in Fig. 2, is made of a material which has a high, constant and uniform secondary electron emission. It has been found that the material which meets these requirements is chromium. The first dynode electrode 24 may be made of solid chromium, e.g. approximately mils thick, or of a chromium layer 28 coated onto a base metal 30. The base metal 30 may be a material such as silver or Nichrome. The chromium coated base structure is preferred due to the fact that a small aperture 17, e.g. 2 to 4 mils, for the electron beam must be drilled in the first dynode, and the chromium coated metal is easier to drill than solid chromium.

The chromium layer 28 may be deposited by any known technique such as sputtering, evaporation, or electrolytic deposition. The chromium layer 28 should be of a thickness sufiicient to prevent electrons from going through the layer when the electrons have energies of approximately 300 electron volts. A layer meeting this requirement is at is substantially inert with respect to the materials used in the manufacture of the photocathode 22 Thus, the combined effects of electron bombardment by the reture beam 15, heat from the cathode of gun 16 and stay materials from the photocathode 22 do not produce dynode burn of the first dynode 24 when the dynode is made of chromium. Furthermore, the secondary electron emission of a chromium first dynode 28, in accordance with this invention, is substantially constant with the age of the tube and is at least as high as that of the materials that had been used prior to this invention.

Although the invention has been described as used in a television pickup tube, it should be understoodthat this invention is also applicable to phototubes of the type including an electron multiplier. An example of a phototube wherein the problem of bad shading is pronounced is the type of phototube that is used in conjunction with a flying-spot scanner for film pickup. type, bad shading produces spurious signals in the output current similar to those previously described.

What is claimed is: I

1. An electron multiplier, said electron multiplier including a dynode having an exposed surface comprising chromium.

2. A device comprising an electron multiplier, said electron multiplier including a first dynode, and said first dynode including an exposed surface of chromium.

3. An electron multiplier including a first dynode, said first dynode including a base material, and a coating of chromium on said base material.

4. An electron multiplier including a first dynode electrode, said first dynode electrode comprising a silver base member, and a coating of chromium on said basemember.

5. An electron multiplier as in claim 4 wherein said coating of chromium is at least approximately .01 mil in thickness.

References Cited in the file of this patent UNITED STATES'PATENTSI Johnson Feb. 21, 1, 950 Lefcourt et al. Dec; 28 1954 OTHER REFERENCES In a tube of this a fr wn J 

